It is now well known to employ acrylate-terminated linear oligomers, especially polyurethane oligomers, in ultraviolet-curing coatings which are applied to optical glass fibers to protect the same. These oligomers, and mixtures thereof with appropriate monoethylenically unsaturated monomers, can be formulated to possess the low modulus at low service temperature needed to avoid microbending difficulties while still possessing sufficient strength at normal temperatures to resist the pressures encountered in cabling the coated fibers. However, these oligomers have been terminated with monoacrylates, such as 2-hydroxyethyl acrylate, and the resulting acrylated oligomers possess only moderate cure speed on exposure to ultraviolet radiation.
It is desired to point out that optical glass fibers are drawn from a specially prepared glass ingot one filament at a time. This means that a rather expensive production apparatus produces very little product per unit of time. To maximize production, it is desired to operate the apparatus as rapidly as possible consistent with the production of a good product. However, it is now possible to operate the glass drawing operation more rapidly than the ultraviolet cure of the applied coating or coatings, and this means that the optical fiber industry is pressuring the coating industry to supply ultraviolet-curing coatings which will cure more rapidly while still providing the combination of moderate modulus at low service temperature with reasonable room temperature strength which is essential to a good optical fiber coating. Moreover, there are other characteristics which a good optical fiber coating should possess, such as a high index of refraction above 1.48.
It is known that polyacrylates, including pentaerythritol triacrylate, can be incorporated into ultraviolet-curing coatings as a reactive diluent. This triacrylate serves many functions, one of which is to enhance cure speed. However, the coatings needed for optical fibers must maintain elastomeric properties at low temperatures, such as -40.degree. C., and preferably as low as -60.degree. C. so as to avoid microbending difficulties. Unfortunately, when a polyacrylate, such as pentaerythritol triacrylate, is added to the acrylate-terminated linear oligomers normally used, it produces harder and less rubbery cured coatings. This causes the tensile modulus at low temperature to increase rapidly as the polyacrylate content increases. As a result, the cured coatings containing the polyacrylate as a reactive diluent are not satisfactory for optical fiber coating.